How can type classes be used to implement persistence, introspection, identity, printing,

Question

In the discussion on The Myths of Object-Orientation, Tim Sweeney describes what he thinks is a good alternative to the all-encompassing frameworks that we all use today.

He seems most interested in typeclasses:

we can use constructs like typeclasses to define features (like persistence, introspection, identity, printing) orthogonally to type constructs like classes and interfaces

I am passingly familiar with type classes as "types of types" but I am not sure exactly how they would be applied to the fore-mentioned problems: persistence, printing, ...

Any ideas?

Answer 1

My best guess would be code reuse through default methods and orthogonal definition through detachment of type class implementation from type itself.

Basically, when you define type class, you can define default implementations for methods. For example Eq (equality) class in Haskell defines /= (not equal) as not (x == y) and this method will work by default for all implementation of the type class. In a similar way in other language you could define a type class with all persistence code written (Save, Load) except for one or two methods. Or, in a language with good reflection capabilities you could define all persistence methods in advance. In practice, it is kind of similar to multiple inheritance.

Now, the other thing is that you do not have to attach the type class to your type in the same place where you define your type, you can actually do it later and in a different place. This allows persistence logic to be nicely separated from the original type.

Some good examples in how that looks like in an OOP language are in my favorite paper ever: http://www.stefanwehr.de/publications/Wehr_JavaGI_generalized_interfaces_for_Java.pdf. Their description of default implementations and retroactive interface implementations are essentially the same language features as I have just described.

_{Disclaimer: I do not really know Haskell so I might be wrong in places}